This is something that I never really paid attention to before and discovered only recently on Youtube: A promo of the Real Ghostbusters cartoon from Time Life.
Originally this was supposed to be a 2 hour Pilot from according to what I read on some forums. But it never got aired for some reason, perhaps due to several issues that Columbia Pictures encountered during approval or during the post-production stage. The pilot never got aired, but scenes from it did find their way into the opening of the show, and as flashback scenes in one of the later episodes. And you'll notice many differences with the character designs of the series that we got to know.
Enjoy!
http://www.youtube.com/watch?v=RqcwocNLH00